A chromium component has been widely used for an element in rust-proofing or surface modification of copper foils for printed wiring boards as plated chromium or treated chromate. Chromate treatment, in particular, has been applied for most of the commercial copper foils at present. A chromium component, when present in the form of chromium compound, is trivalent or hexavalent in terms of oxidation number. And hexavalent chromium is much more toxic in biological and easier to diffuse in the soil.
For these reasons, EU has adopted the ELV Directive forbidding use of environmental burden substances (lead, hexavalent chromium, mercury and cadmium) in new vehicles registered in the EU markets on and after Jul. 1, 2003. Together recommending positive use of trivalent chromium. Moreover, the EU's WEEE (Waste Electrical and Electronic Equipment) Directive and RoHS (Restriction on Hazardous Substances) Directive are finally agreed by the electric/electronic industries. These directives restrict use of a total of 6 substances including hexavalent chromium (Cr6+) as the specific hazardous substances used in waste electric/electronic equipments which may include environmental risks, even when recovered separately from discarded electric/electronic devices, and printed wiring board is included in them.
On the other hand, transportation of hazardous wastes over country boundaries have been more noted worldwide since the 1970s. Since the beginning of the 1980s, the environmental pollution problems resulting from hazardous wastes transported from developed countries and left in developing countries caused environmental pollution more serious. As a result, “Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal” was adopted in which the international frameworks and procedures for restricting transboundary movements and the like of specific wastes was specified. It has been valid in Japan since 1993.
In addition, by increasing awareness of environmental issues, trivalent chromium may be converted into the hexavalent state with inadequate waste treatment, or it may be judged to be hexavalent by inadequate analytical procedures. In such background, applying of chromium free copper foils for printed wiring boards has been studied.
Patent Document 1 discloses idea on a metallic foil with an adhesion-promoting layer which contains at least one kind of silane coupling agent on at least one side which is characterized in chromium-free. And the disclosed chromium-free copper foil is also characterized in that the formed base metallic foil surface below the adhesion-promoting layer is neither roughened, nor coated with a zinc or chromium layer. The document also describes a metallic layer provided between the metallic foil surface and adhesion-promoting layer, the metallic layer containing a metal selected from the group consisting of indium, tin, nickel, cobalt, brass, bronze and a mixture of two or more of these metals, or from the group consisting of tin, chromium/zinc mixture, nickel, molybdenum, aluminum and a mixture of two or more of these metals.
Patent Document 2 discloses a copper foil for printed wiring board characterized in that a copper foil is coated with a metallic or alloy layer composed of at least one species of metal selected from the group consisting of nickel, molybdenum, cobalt and zinc, a coupling agent layer is formed on the metal or alloy layer, and an adhesion enhancing layer containing a linear polymer are applied in this order to provide an environment-friendly electrodeposited copper foil for printed wiring boards free from hazardous chromium.    Patent Document 1: Japan Patent Laid Open 07-170064    Patent Document 2: Japan Patent Laid Open 2004-47681
However, the invention disclosed in Patent Document 1 lacks in definitions judging from the whole description. So, it is unfortunately considered to be almost unrealizable except for those described in the embodiments. Moreover, it fails to realize a fully chromium-free copper foil, because applying of a combination of zinc and chromium for a stabilizer layer is described in the examples although it is aiming to provide a chromium-free copper foil.
Patent Document 2 discloses properties of a copper clad laminate, made of the copper foil for printed wiring boards and FR-5 grade prepreg (GEA-679N) in the table. The table describes peel strength and peel loss after hydrochloric acid dipping. However, circuit width to examine these properties is not clear. So, the present inventors have prepared the copper foil for tracing of the description of the document, and applying FR-4 grade prepreg which is most popularly used for manufacturing printed wiring boards to examine peel strength with fine circuits of 1 mm width or less. As a result, it was found that the copper foil is not suitable for fine-pitch circuits because peel loss after both hydrochloric acid dipping and moisture absorption is quite big.
As it is clear from above discussion, the layer containing such a metal is commonly referred to as a rust-proofing layer, a metallic component except copper applied on a copper foil performs to keep long-term shelf life by protecting copper foil from oxidation with ambient air. However, a rust-proofing layer may change performance of adhesion to a resin substrate depending on layer type. Especially after fabricating a printed wiring board, it affects on basic properties, peel strength, peel loss after both chemical treatments and moisture absorption, solder blister and the like.
Therefore, there are requirement on surface-treated copper foil without chromium in the rust-proofing layer which performs sufficient peel strength for wirings, less peel loss after both chemical treatments and moisture absorption and good solder blister after fabricating printed wiring board.